The 27-year-old is keen on a move back to Italy.
His agent Paolo Fabbri said: “We have undergone negotiations with Boro for the player’s free transfer to Siena.
“There is an agreement with Siena and Maccarone could sign a three or four-year contract with them.”
Middlesbrough paid a club record £8.15 million when he joined them from Empoli in 2002.
